Spherulitic crystal morphology

The two sketches in Fig. 5.61 illustrate different paths to a spherulitic crystal morphology. These paths create a spherical overall appearance, despite the fact that the basic crystals are polyhedra, i.e., the spheruhtes are aggregates of polyhedral... 

The discussion above shows that structural problems in CPs are very different from those in organic conductor crystals. They also differ somewhat from those in other polymers the dominant polymer crystal morphologies, lamellae and spherulites [12] are not found. Rather, fibrils are often observed. 

Dilatometric data by measurement of stress decay on fibers are shown in Fig. 3.103 for extension ratios a from one (no extension) to six. Besides the increase in rate of crystallization on stretching, one finds a decrease of the Avrami exponents from 3.5 to 1.3 with increasing extension ratio. The nature of the crystal morphology is changing from spherulitic to fibrillar as a increases. 

The properties of PLA, as indeed those of other polymers, depend on its molecular characteristics, as well as on the presence of ordered structures, such as crystalline thickness, crystallinity, spherulite size, morphology and degree of chain orientation. The physical properties of polylactide are related to the enantiomeric purity of the lactic acid stereo-copolymers. Homo-PLA is a linear macromolecule with a molecular architecture that is determined by its stereochemical composition. PLA can be produced in a totally amorphous or with up to 40 per cent crystalline. PLA resins containing more than 93 per cent of L-lactic acid are semi-crystalline, but, when it contains 50-93 per cent of it, it is entirely amorphous. Both meso- and D-lactides induce twists in the very regular PLLA architecture. Macromolecular imperfections are responsible for the decrease in both the rate and the extent of PLLA crystallization. In practise, most PLAs are made up of L-and D,L-lactide copolymers, since the reaction media often contain some meso-lactide iir turities. 

Fig. 24. DTA melting peak temperature as a function dt heating rate for poly-ethylenes of different crystal morphologies. Curve 1, attended diain crystak curve 2, slowly cooled melt crystallized spherulites curve 3, fast cooled mdt crystallized spherulites curve 4, solution grown folded chain single crystals.

Melt Crystallized.—Spherulites are observed to consist of radiating fibrils or lamellae which in melt-crystallized polymers are 100—500 A thick, depending on crystallization temperature, and /um in breadth. The c axis, i.e, the chain direction, coincides with the thickness of the lamellae and, by analogy to single crystal morphology, the molecular chain folds backwards into the lamellae. Amorphous polymer fills the regions between lamellae and can account for a substantial amount of the content. Painter et al. have shown that these inter-lamellar regions are very similar to melt-crystallized polymer in their i.r. spectrum. 

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